5.7 NAMES & FORMULASOF IONIC COMPOUNDS

The International Union of Pure & Applied Chemistry (IUPAC)

• Decides on a common naming system known by scientists across the world

• Recall—Ionic Compounds:2 elements; a metal & a non-metal

• Therefore their names have 2 parts

first part metal ion

second part non-metal ion

• The first part of the name is the same as the name of the neutral metal atom

• The second part of the name changes the non-metal ending to “ide”

Examples

• magnesium + chlorine = magnesium chloride

(metal) (non-metal) (ionic compound)

• aluminum + oxygen = aluminum oxide (metal) (non-metal) (ionic compound)

Writing these chemical formulas.

• Remember… electrons move from metal atoms to non-metal atoms

• The charged ions attract other ions of the opposite charge until the charges balance out (positive + negative = zero)

• The compound that forms is electrically neutral

*** # of positive charges = # of negative charges***

Chemical Formula of Magnesium Chloride

o 1 Write the symbols of the elements, with the metal on the left and the non-metal on the right

Mg Cl

o 2 Add the ionic charge of each ion above the symbol +2 -1 Mg Cl

o 3 Determine how many ions of each are required to bring the total charge to zero.

1 (+2) + 2(-1) = 0

Mg Cl

Magnesium Chloride Continued…

o 4 Write the chemical formula using the coefficients in front of each as the subscripts

MgCl2 magnesium chloride

***Do not write the subscript “1” in chemical formulas because the symbol itself represents

one ion ***

CRISS CROSS RULE(An easier way to write chemical formulas)

Ex: Magnesium Chloride +2 -1

• 1 Write the symbols for each element MgCl

• 2 Write the ionic charge above each symbol (the # e- gained/lost to become an ion)• 3 Drop the +/- sign• 4 Criss cross the numbers of the ionic charges• 5 Write the numbers as subscripts : MgCl2• 6 Do not write a 1 as a subscript as the 1 is

already assumed

Sometimes………

• Occasionally you get an answer with two identical subscripts

• Ex: Al3N3

• Write the compound with subscripts in lowest terms

• Ex: AlN

This is what it all means!

MgCl2There is one atom of magnesium in the molecule

There are two atoms of chlorine in the molecule

Magnesium has two electrons to give away. Chlorine only needs 1 electron. There fore two chlorines are needed to take the two available electrons from magnesium.

Elements with Multiple Charges

• Some elements have two stable cations.e.g. Fe2+ and Fe3+

• Fe2+ is called Iron (II) and Fe3+ is called Iron (III)

• There are two different compounds of iron and chlorine

FeCl2 (light green solid) and FeCl3 ( yellow-brown solid)

• Named the same way, except insert rounded brackets with Roman numerals indicating the ionic charge

Some other Elements with Multiple Ionic Charges

Metal Chemical symbol of element

Chemical Symbols of ions

Names of Ions

copper Cu Cu+

Cu2+

copper (I)copper (II)

Iron Fe Fe2+

Fe3+

Iron (II)Iron(III)

Lead PB Pb2+

Pb4+

lead (II)lead (IV)

Manganese Mn Mn2+

Mn4+

manganese (II)Manganese (IV)

tin Sn Sn2+

Sn4+

tin(II)tin(IV)

Naming these Compounds

• Whenever you name a compound containing a metal, ALWAYS check to see if the metal has more than one ionic charge.

• metal ( Roman Numeral ) Non metal (with “ide”)

• If it does, use reverse criss cross to determine the correct “bracket”

3+ 1-

FeCl3• Double check that the non metal has the correct ionic charge as

well

• If so then it is Fe3+ and the compounds name is iron (III) chloride

Another Example FeP1 1

FePBut iron’s valence is 2 or 3 and phosphorous’ valence is 3,5 or 4. Therefore the common valence is 3.

3+ 3-

FeP

Iron (III) Phosphide

Homework

• Two Handouts

• Textbook: pg 200 # 2-5, 7, 9